The present invention pertains to apparatus and a process for simultaneously sampling the ambient atmosphere and for providing a calibration sample of the ambient atmosphere with a determinable amount of a pollutant normally present in the ambient atmosphere.
The measurement of trace amounts of pollutants in the ambient atmosphere has become of significant importance since there is some evidence that human exposure over a prolonged period of time to certain gases and volatiles that might be present in the ambient atmosphere may be detrimental to one's health. As a consequence of this finding, the government has imposed limitations on the amounts of certain materials that are permitted in the ambient atmosphere to which a person is subjected. Such limitations may provide for the presence of only a few "parts-per-million" of a particular pollutant measured over a specified period of time. The government requirements have made it necessary to at least periodically, if not essentially continuously, monitor the ambient atmosphere to make certain that pollutants that might be present in the ambient atmosphere at a particular location are maintained within "acceptable" levels.
A conventional analytical process for measuring the quantity of a pollutant in the ambient atmosphere over a specified period of time involves flowing ambient atmosphere at a known rate through a collector to be filled with activated charcoal particles for the prescribed period of time and absorbing the pollutant on the activated charcoal within the collector tube. The collector tube then is placed in a heater in which the collector tube is rapidly heated to cause the pollutant to be desorbed or "flashed" from the collector tube. The "flashed" pollutant sample is introduced into a gas chromatograph analyzer in which the quantity of the pollutant obtained from the collector tube is measured. The average concentration of the pollutant present in the ambient atmosphere during the measured time period is reported as "parts of pollutant per quantity of ambient temperature" (for example, "parts of pollutant per million parts of ambient atmosphere"). The accuracy of the test results depends primarily upon the efficiency of the collector tube in entrapping pollutant from the ambient atmosphere and in the effectiveness of releasing the pollutant from the collector tube during the "flashing" step.
It has been found that the humidity of the ambient atmosphere during the period of sampling significantly affects the accuracy of the test results. When sampling is done during a period in which the ambient atmosphere contains a high amount of moisture, some of the pollutant becomes desorbed by the moisture vapor and passes through the collector tube. Also, during a high humidity period the moisture vapor in the ambient atmosphere can occupy sites of the activated carbon within the collector tube that normally would "entrap" pollutant. The ability of the activated carbon to remove pollutant from the ambient atmosphere as a result is reduced and pollutant is permitted to pass through the collector tube without being entrapped on the activated carbon. The test results obtained, as a consequence, will indicate a lesser quantity of the pollutant in the ambient atmosphere than actually is present during the measured time interval.